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I was watching a campaign video from 2000, in it Bush clearly thinks the USA's position as the 'world police' or that their foreign intervention strategy, especially ones that instill virtues on other sovereign states, are ill advised.

How is this reconciled with many choices he made as president, namely the Iraq war? Why did he change his tune?

I would like to point out that while Bush himself did make such statements, many of the key figures in his cabinet made completely opposite statements.

10 of his cabinet members, including central ones such as Dick Cheney, Donald Rumsfeld, and Paul Wolfowitz, were signatories to the mission statement of a think-tank called The Project for a New American Century. This group had 25 signatories in total.

PNAC's stated goal was "to promote American global leadership." The organization stated that "American leadership is good both for America and for the world," and sought to build support for "a Reaganite policy of military strength and moral clarity."

Observers such as Irwin Stelzer and David Grondin have suggested that the PNAC played a key role in shaping the foreign policy of the Bush Administration, particularly in building support for the Iraq War.

Key PNAC members had been advocating regime change in Iraq since 1998.

One particular publication by PNAC which is discussed frequently is Rebuilding America's Defenses: Strategy, Forces and Resources For a New Century, published in September 2000. Here are some quotes from it which I consider signifigant:

The immediate task is to rebuild today's force, ensuring that it is
equal to the tasks before it: shaping the peacetime environment and
winning multiple, simultaneous theater wars; these forces must be
large enough to accomplish these tasks without running the “high” or
“unacceptable” risks it faces now.

None of the defense reviews of the past decade has weighed fully
the range of missions demanded by U.S. global leadership: defending
the homeland, fighting and winning multiple large-scale wars,
conducting constabulary missions which preserve the current peace,
and transforming the U.S. armed forces to exploit the “revolution
in military affairs.”

Further, the process of transformation, even if it brings
revolutionary change, is likely to be a long one, absent some
catastrophic and catalyzing event – like a new Pearl Harbor.

Emphasis mine. I do not mean to imply anything by the last quote. Being one of the more famous quotes from the document, however, I felt I had to include it.

I'd also like to point out that politicians do not always mean what they say, both during campaigns, elections, and their terms in office.

Whether or not Bush truly did agree with the sentiments of PNAC before 9/11, the fact that he selected so many of its signatories for his cabinet is, in my opinion, more of a political statement than anything he said during his campaign. Actions speak louder than words. To give Bush the benefit of the doubt, it's possible that he simply did not know his cabinet members very well. That would be grossly incompetent, which is certainly possible. I do not speculate on his personal character or motives.

Sources for claims about PNAC are in their own mission statement, and in the sources of the Wikipedia article on PNAC. Two sentences in this answer have been copied verbatim from Wikipedia.

Sept. 11 changed everything. It demonstrated with the utmost clarity the risks of an unpoliced world.

The first change was in Afghanistan. The Taliban had largely ruled Afghanistan since about 1996. The US certainly didn't like them, but also didn't view them as America's problem.

On September 11, the NSA and German intelligence both intercepted communications pointing to Bin Laden. Some hijackers were identified while the planes were still in the air. Mohamed Atta's luggage, which did not make his flight, was also helpful.

Already on Semptember 12, many in the US suspected that he was to blame.

It also helped that the hijackers purchased their tickets using their real names. By September 14, the FBI released the names of the hijackers.

Also on Sept. 14, Congress passed a law authorizing the military to attack anyone associated with terrorists. The vote was 98-0 in the Senate and 420-1 in the House.

On Sept. 21, the Taliban refused a US demand to hand over Bin Laden. For Bush, that was plenty of reason to go to war. The US decision to overthrow the Taliban was supported by nearly everyone in the US.

Then there was Iraq. Although he was not involved in planning them, Saddam Hussein originally praised the attacks. Saddam had used chemical weapons during the war against Iran, and he was refusing to cooperate with inspectors who were supposed to verify that he had given them up. Some people reasoned, wrongly, that if he didn't have anything to hide, he would be cooperating with the inspectors. And Saddam had a history of supporting terrorists, for example paying $25,000 to the family of every Palestinian suicide bomber.

Based on this, Bush made a case for war against Saddam. That was hotly debated at the time. Most Republicans were in favor. Some Democrats, such as Hillary Clinton, were also in favor, while others, such as Barack Obama, were opposed. United Republican support and mixed Democratic sentiment was enough to get Congress to agree to the invasion.

During his presidential campaign GW Bush promised the kind of foreign policy that (he thought) would get him elected in 2000. Subsequently, he enacted a foreign policy that (he thought) would get him re-elected in 2004. The striking difference between the two is probably due to changing attitudes of the US electorate after the 9/11 terrorist attacks. One additional explanation of politicians doing different things when in power versus campaign promises is to look for sponsors who the politicians "owe" favours in return for financial support of their campaign.